In free-space (FS) QKD, signals are sent via line-of-sight transmission, e.g. between two ground stations or between a satellite and ground station. Satellite-based FS-QKD has attracted a great deal of attention due to its potential to allow QKD over global distances[1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}. In contrast, ground based FS-QKD typically operates over short to medium distances, ideal for deployment in dense urban spaces where it may be difficult and costly to deploy ad hoc short range fibre links. FS-QKD between either buildings or existing towers should allow secure communication in a more flexible manner.
A prototype of such a network is being constructed as part of the AirQKD project [14]}. Here, we present high-level analysis of terrestrial FS-QKD operational trade-offs with particular consideration of how finite key statistical effects and uncertainties impact upon their design.
